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Optimal Thermostat Settings Two Zone HVAC?
Old 12-03-2019, 10:20 AM   #1
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Optimal Thermostat Settings Two Zone HVAC?

We recently moved into a house with a one AC and one furnace but two zones with two programmable thermostats (and the associated active damper, controller etc.). I’m trying to figure out the most efficient settings for upstairs and downstairs assuming we don’t care about the upstairs temperature. Goal is comfortable downstairs, upstairs can float for overall efficiency as upstairs is only storage.
  • All sources we’ve read say the same temp on both isn’t efficient.
  • All sources say don’t close off the upstairs unless your heating and cooling is sized to only handle one zone or the other versus whole house. Makes sense from what I’ve read, please do a search if you want to know more.
  • Some online experts recommend always set upstairs 2F warmer than main floor whether heating or cooling.
  • Some say set upstairs 2F warmer than downstairs for cooling, and 2F cooler when heating. One source suggested a 4F delta.
Since warm air always rises, the last recommendation make sense to me. Again we only care about main floor comfort and overall efficiency - e.g. if upstairs is colder in winter and warmer in summer for the sake of lower utility usage great.

Any of you with similar HVAC zoned systems and years of experience know?
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Old 12-03-2019, 10:30 AM   #2
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We have 2 completely separate systems for upstairs and downstairs, so our experience may not be helpful. I will just say that for a couple of years the upstairs furnace did not work, so we never had heat upstairs in the winter. Here in Texas the outside temps get down below freezing about 4 times each winter and last night temps were in the high 30s.

Anyways, for those 2 winters, our heating bills were lower and the upstairs never got colder than about 64 degrees.

I'd say just set the upstairs to the lowest possible temperature that you can tolerate for the winter heating season.

We have a very good site plan for our home with oaks and sweet gum shielding the south side of the house with their leaves in the summer, but those leaves fall off in the winter and expose the south side to the warming rays of the sun.
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Old 12-03-2019, 10:37 AM   #3
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Part of it depends on how open your house is. More open means more heat will rise.

I have two separate units, one for up, one for down. During the day I turn the heat way down upstairs. I only turn it up (by program) an hour or so before bedtime, and an hour or so before I wake. Unless it's really cold, it rarely comes on.

I would set the upstairs temp at 55, just to make sure any pipes you might have up there don't freeze. I bet it never hits that unless you are away.

I don't have enough experience with cooling. If it were me, I'd probably set the upstairs temp to whatever you want for the storage, and no lower.
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Old 12-03-2019, 10:49 AM   #4
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We also have one unit but 3 zones on a single story house. I have noticed that if we run all 3 zones, it's costs significantly more...but not really sure why this is (it is a geothermal/ground source heat pump). So, we tend to run only one zone at a time and the rest of the house stays comfortable enough.
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Old 12-03-2019, 12:59 PM   #5
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I'd suggest you call a HVAC company in your area, that will understand your climate. Describe your system to them and ask for their thoughts. There may be too many variables across the members here (heating system, 1 or 2 units, heat pump vs. gas vs. oil vs. electric, hot air vs hot water, etc..
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Old 12-03-2019, 02:30 PM   #6
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We're in the northeast, with two systems, two thermostats. It is a challenge, but careful observation helps. The placement of the thermostats is important. For example, ours are positioned on two floors, one above the other. First floor is in my office, with double doors. Second floor thermostat is in a hallway.

Our situation has a large open space with stairway from 1st to 2nd floor. This space has great effect, and it took years to understand what the best settings were, and how settings need tweaking in each season.

In winter, when I'm home, it's 68 or 69 setting on 1st floor, 63 setting on 2nd floor. Not home, 63 both.

Each situation is different, of course. Knowing where thermostats are located, and other characteristics of home would help.
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Old 12-03-2019, 03:14 PM   #7
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This just dawned on me... the local utility may come out for free and do an energy audit on your home and help answer the question.
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